It is known that surface states on a GaAs device can trap minority carriers and degrade device characteristics.
The surfaces of GaAs devices must be passivated before they are useful since the minority carriers are trapped by the surface states and recombine. The surface is conventionally passivated with an epitaxially grown layer of GaAlAs which is lattice matched to the GaAs and stops the minority carrier diffusion to the GaAlAs surface.
In the background prior art, U.S. Pat. No. 4,058,413 by Welch et al., shows the use of sputtered AlN for passivation; and U.S. Pat. No. 3,705,059 by Kun, shows the use of layers containing constituents useful in later heat treating steps.
The photoluminescent properties of GaAs have been extensively studied in the past. Most of these studies have dealt with attempts to associate a variety of optical transitions with a corresponding variety of dopants and defects in GaAs. In these previous studies, it was found that the variation of short circuit current with excitation photon energy provided information about minority diffusion lengths, and interface and surface recombination velocities.